Compute clusters employing photonic interconnections for transmitting optical signals between compute cluster nodes

ABSTRACT

Various embodiments of the present invention are directed to photonic-interconnection-based compute clusters that provide high-speed, high-bandwidth interconnections between compute cluster nodes. In one embodiment of the present invention, the compute cluster includes a photonic interconnection having one or more optical transmission paths for transmitting independent frequency channels within an optical signal to each node in a set of nodes. The compute cluster includes one or more photonic-interconnection-based writers, each writer associated with a particular node, and each writer encoding information generated by the node into one of the independent frequency channels. A switch fabric directs the information encoded in the independent frequency channels to one or more nodes in the compute cluster. The compute cluster also includes one or more photonic-interconnection-based readers, each reader associated with a particular node, and each reader extracting the information encoded in the independent frequency channels directed to the node for processing.

TECHNICAL FIELD

The present invention relates to compute clusters, and, in particular,to photonic-interconnection-based compute clusters that include opticaltransmission paths for transmitting data encoded in frequency channelsof an optical signal between compute cluster nodes.

BACKGROUND OF THE INVENTION

Recent developments in distributed computing platforms, called “computeclusters,” show that considerable progress has been made in increasingdata processing speed and reducing platform sizes. In general, a computecluster comprises a number of interconnected nodes, each of which iscapable of performing a number of independent data processing tasks. Anode can be a processor, memory, computer server, storage server, anexternal network connection or any other data processing or datatransmitting device. Compute clusters are typically employed in eitherdata reduction applications or data generation applications. In datareduction applications, large input data sets, such as data provided bya scientific instrument, are processed by identifying patterns and/orproducing aggregate statistical descriptions of the input data. Forexample, in order to analyze and interpret the large amounts of imagedata obtained from an optical scan of a microarray, the image data canbe reduced to smaller aggregate statistical descriptions. In datageneration applications, small input data sets typically provide initialconditions for simulations that generate large output data sets that canbe further analyzed or visualized. Combustion models, weatherprediction, and computer graphics applications that generate animatedfilms are examples of data generation applications.

Compute cluster applications are typically partitioned into hundreds,thousands or even millions of tasks by identifying specific individualtasks that can each be independently performed. Applications are oftenpartitioned by a message-passing interface computer program andexecution environment. Tasks can be distributed to different nodes basedon the following criteria: (1) the order in which each task is received,(2) the configuration of the nodes in the cluster, (3) the computationaldemand of each task, (4) the amount of memory needed for each task, (4)the amount of data transmitted between nodes, and (5) the input/outputrequirements of the application.

Compute cluster nodes are typically interconnected via a network ofhigh-speed, low latency, electrical interconnections that transmit databetween nodes through a switch fabric. FIG. 1A illustrates arepresentation of a 4-node switch-fabric architecture 100. In FIG. 1A,physical nodes are interconnected via a switch fabric 102, where eachphysical node is represented by a first virtual node and a secondvirtual node. The first virtual node represents an input connection withthe switch fabric 102, and the second virtual node represents an outputconnection with the switch fabric 102. For example, an input connectionbetween physical Node 0 and the switch fabric 102 is represented by arectangle 104 and a directional arrow 106, and an output connectionbetween the switch fabric 102 and the physical Node 0 106 is representedby a rectangle 108 and a directional arrow 110. Switch fabrics provideinterconnections so that nodes can simultaneously transmit data todifferent nodes in the compute cluster. For example, the switch fabric102 provides interconnections so that the Node 1 can be simultaneouslytransmit data to the Node 2 and the Node 3, as indicated by dashed-linedirectional arrows 112 and 114.

The data processed by each node is typically partitioned into smallerfixed-sized packets that are then distributed through the switch fabricto particular nodes for processing. FIG. 1B illustrates an exampleimplementation of the switch fabric 102, shown in FIG. 1A. In FIG. 1B,the switch fabric 102 includes input and output line cards, such as aninput line card 118 and output line card 120, a permutation network 122,and an arbiter 124. Data is first transmitted from the nodes to theinput line cards. The input line card partitions the data streams intofixed size packets. The packets are then transmitted to the switchfabric 102 and distributed to one or more first-in-first-outelectronic-based data structures called “virtual queues.” The arbiter124 receives information regarding the packets stored at the head ofeach virtual queue and accordingly configures the interconnectionswithin the permutation network 122 to distribute a first batch ofpackets stored at the head of each virtual queue to particular nodes.The output line cards assemble the packets received by the permutationnetwork 120 and transmit the assembled packets to nodes for processing.After the arbiter 124 has distributed the first batch of packets, thearbiter 124 reconfigures the permutation network 120 in order todistribute a second batch of packets stored at the head of each virtualqueue for processing.

In general, switch fabrics uniformly distribute data between nodes.However, compute clusters often have a number of nodes that exchangelarge amounts of data more frequently than other nodes, and the lowlatency interconnections provided by switch fabrics have limitedbandwidths. As a result, the amount of data that can be transmittedbetween nodes is not well matched to the data transfer needs of theparticular nodes at each point in time, resulting in data processingdelays. In addition, arbiters can delay data processing, becausearbiters typically rely on receiving information regarding all packetslocated at the head of each queue before distributing a batch ofpackets. Manufacturers, designers, and users of compute clusters haverecognized a need for an interconnection architecture that provideslarge bandwidth, high-speed interconnections, and a switch fabric thatdoes not rely on an arbiter to distribute packets.

SUMMARY OF THE INVENTION

Various embodiments of the present invention are directed tophotonic-interconnection-based compute clusters that provide high-speed,high-bandwidth interconnections between compute cluster nodes. In oneembodiment of the present invention, the compute cluster includes aphotonic interconnection having one or more optical transmission pathsfor transmitting independent frequency channels within an optical signalto each node in a set of nodes. The compute cluster includes one or morephotonic-interconnection-based writers, each writer associated with aparticular node, and each writer encoding information generated by thenode into one of the independent frequency channels. A switch fabricdirects the information encoded in the independent frequency channels toone or more nodes in the compute cluster. The compute cluster alsoincludes one or more photonic-interconnection-based readers, each readerassociated with a particular node, and each reader extracting theinformation encoded in the independent frequency channels directed tothe node for processing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a representation of a 4-node switch-fabricarchitecture.

FIG. 1B illustrates an example implementation of the switch fabric shownin FIG. 1A.

FIG. 2 illustrates an example of a one-dimensional photonic crystal.

FIG. 3 illustrates an example of a two-dimensional photonic crystal.

FIGS. 4A-4B are hypothetical plots of frequency versus wave vectorz-component for a first one-dimensional photonic crystal and a secondone-dimensional photonic crystal, respectively.

FIGS. 5-6 illustrate perspective views of two two-dimensional photoniccrystals.

FIGS. 7A-7B illustrate propagation of a transverse electric field andmagnetic field modes in the two-dimensional photonic crystal shown inFIG. 5.

FIG. 8 illustrates a photonic band structure of transverse electricfield and magnetic field modes propagating in the two-dimensionalphotonic crystal shown in FIG. 5.

FIG. 9 illustrates an example of a photonic crystal with two resonantcavities and a waveguide.

FIG. 10 is a hypothetical plot of frequency versus the magnitude of wavevector for the waveguide of the photonic crystal shown in FIG. 9.

FIGS. 11A-11E illustrate examples of information encoded inelectromagnetic signals.

FIG. 12 illustrates a photonic-interconnection-based compute clusterthat represents one of many embodiments of the present invention.

FIGS. 13A-13C illustrate examples of waveguides in a photonic crystalthat can be used to transmit and distribute an optical signal to eachgroup of nodes in a compute cluster, each representing one of manyembodiments of the present invention.

FIG. 14 illustrates an example of a group of nodes that represents oneof many embodiments of the present invention.

FIG. 15A illustrates a photonic-crystal-based writer that encodes datain a specific frequency channel of an optical signal and that representsone of many embodiments of the present invention.

FIG. 15B illustrates a photonic-crystal-based reader that extracts dataencoded in a specific frequency channel of an optical signal and thatrepresents one of many embodiments of the present invention.

FIG. 16A illustrates a resonant cavity that can be used as either a dropfilter or an add filter and that represents one of many embodiments ofthe present invention.

FIG. 16B illustrates a first configuration of a detector/modulator thatrepresents one of many embodiments of the present invention.

FIG. 16C illustrates a second configuration of a detector/modulator thatrepresents one of many embodiments of the present invention.

FIG. 17 illustrates a schematic representation of the switch fabric,shown in FIG. 12, that represents one of many embodiments of the presentinvention.

FIG. 18 illustrates an exemplary implementation of an electronic-basedswitch fabric that represents one of many embodiments of the presentinvention.

FIG. 19A illustrates an example of a cyclic permutation network thatrepresents one of many embodiments of the present invention.

FIG. 19B illustrates four possible cyclic permutation outputs that canbe generated by the cyclic permutation network shown in FIG. 19A andthat represents one of many embodiments of the present invention.

FIG. 20 illustrates an implementation of a hypothetical virtual outputqueue that represents one of many embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the present invention are directed tophotonic-interconnection-based compute clusters that provide high-speed,high-bandwidth photonic interconnections for transmitting data with acompute cluster. The photonic interconnections include opticaltransmission paths for transmitting an optical signal from an opticalsignal source to one or more compute cluster nodes. The optical signalincludes numerous, independent electromagnetic waves called “frequencychannels” that can each be separately modulated to encode data that canthen be directed to particular nodes in the compute cluster. Each nodeis connected to a first filter that extracts a first frequency channelencoding data directed to the node, and each node is connected to asecond filter that can be used to encode data directed to a differentnode in the compute cluster by modulating a second frequency channel. Aswitch fabric can be included to direct data through frequency channelsto nodes. A single external optical source can be used to generate theoptical signal, avoiding separate, node-associated optical-signalsources.

The present invention is described below in the subsections: (1) anoverview of photonic crystals and waveguides, (2) an overview ofencoding data in electromagnetic waves, and (3) embodiments of thepresent invention.

An Overview of Photonic Crystals and Waveguides

Photonic crystals are optical devices composed of two or more differentmaterials with dielectric properties that, when combined together in aregular pattern, can modify the propagation characteristics ofelectromagnetic radiation (“ER”). FIGS. 2 and 3 illustrate two of manydifferent possible patterns in which two different materials withdifferent dielectric properties can be combined to form a photoniccrystal. Photonic crystals are typically identified by the number ofdirections in which the dielectric pattern is periodic. For example,FIG. 2 illustrates an example of a one-dimensional photonic crystal. InFIG. 2, a photonic crystal 200 is composed of seven layers of twodifferent dielectrics that alternate periodically in the z-direction.Unshaded layers 201-204 are composed of a first dielectric having adielectric constant ∈₁, and hash-marked layers 205-207 are composed of asecond dielectric having a different dielectric constant ∈₂. The layersare regularly spaced with a repeat distance called a “lattice constant,”in the case of the lattice constant shown in FIG. 2, lattice constant a.FIG. 3 illustrates an example of a two-dimensional photonic crystal. Thetwo-dimensional photonic crystal 300 comprises alternating layers of twodifferent dielectrics, and is periodic in both the x-direction and they-direction with two lattice constants a and b. Unshaded regions, suchas region 301, are composed of a first dielectric having dielectricconstant ∈₁, and hash-marked regions, such as region 302, are composedof a second dielectric having a different dielectric constant ∈₂.Photonic crystals can also be fabricated with repeating patterns inthree dimensions. Three-dimensional photonic crystals can be fabricatedusing spheres, tubes, or other solid shapes comprising a firstdielectric embedded in a slab comprising a second dielectric.

ER propagating in a dielectric can be characterized by electromagneticwaves comprising oscillating, orthogonal electric fields, {right arrowover (E)}, and magnetic fields, {right arrow over (H)}, and a directionof propagation, {right arrow over (k)}. The electric and magnetic fieldsare related by Maxwell's equations: $\begin{matrix}{{{Equation}\quad 1\text{:}}{{\bigtriangledown \cdot {\overset{\_}{H}\left( {\overset{\_}{r},t} \right)}} = 0}} & \quad \\{{{Equation}\quad 2\text{:}}{{{\bigtriangledown \cdot {ɛ\left( \overset{\_}{r} \right)}}{\overset{\_}{E}\left( {\overset{\_}{r},t} \right)}} = 0}} & \quad \\{{{Equation}\quad 3\text{:}}{{\bigtriangledown \times {\overset{\_}{E}\left( {\overset{\_}{r},t} \right)}} = {- \frac{\partial{\overset{\_}{H}\left( {\overset{\_}{r},t} \right)}}{\partial t}}}} & \quad \\{{{Equation}\quad 4\text{:}}{{\bigtriangledown \times {\overset{\_}{H}\left( {\overset{\_}{r},t} \right)}} = {{ɛ\left( \overset{\_}{r} \right)}\frac{\partial{\overset{\_}{E}\left( {\overset{\_}{r},t} \right)}}{\partial t}}}} & \quad\end{matrix}$where {right arrow over (r)} is spatial displacement of anelectromagnetic wave in the dielectric, t is time, and ∈ ({right arrowover (r)}) is a dielectric constant.

Because dielectrics do not generally support free charges or freecurrents, Equations 1-4 do not include a charge density term or a volumecurrent density term. Equations 3 and 4, the curl equations, are lineardifferential equations. In both equations, the left sides express thedependence of a field on the independent spatial displacement {rightarrow over (r)}, and the right sides express the dependence of a fieldon t. The only way for a differential quantity that varies with respectto {right arrow over (r)} to remain equal to a quantity that varies withrespect to t, is for the differential quantities to equal the sameconstant value. Both sides of Equations 3 and 4 are equal to a constant,and the method of separation of variables can be applied to obtain:{right arrow over (H)}({right arrow over (r)},t)={right arrow over(H)}({right arrow over (r)})exp(iωt){right arrow over (E)}({right arrow over (r)},t)={right arrow over(E)}({right arrow over (r)})exp(iωt)where ω is the angular frequency of an electromagnetic wave propagatingin a dielectric.

Maxwell's curl Equations 3 and 4 can be decoupled by dividing Equation 4by the dielectric constant ∈ ({right arrow over (r)}), applying the curloperator, and substituting Equation 3 for the curl of the electric fieldto give: $\begin{matrix}{{{Equation}\quad 5\text{:}}{{\Theta\quad{\overset{\_}{H}\left( \overset{\_}{r} \right)}} = {\omega^{2}{\overset{\_}{H}\left( \overset{\_}{r} \right)}}}{{{where}\quad\Theta} = {\bigtriangledown \times \left( {\frac{1}{ɛ(r)}\bigtriangledown \times} \right)\quad{is}\quad a\quad{differential}\quad{{operator}.}}}} & \quad\end{matrix}$

Equation 5 is an eigenvalue equation, where the eigenvalues are ω², andthe eigenfunctions are the corresponding magnetic fields {right arrowover (H)}({right arrow over (r)}). After the magnetic fields {rightarrow over (H)}({right arrow over (r)}) are determined according toEquation 5, the electric field {right arrow over (E)}({right arrow over(r)}) can be obtained by substituting {right arrow over (H)}({rightarrow over (r)},t) into Equation 3 and solving for {right arrow over(E)}({right arrow over (r)}).

For finite dimensional photonic crystals, such as the photonic crystalsshown in FIGS. 1 and 2, the eigenvalues and eigenfunctions of Equations5 are quantized to give:Θ{right arrow over (H)} _(j)({right arrow over (r)})=ω_(j) ² {rightarrow over (H)} _(j)({right arrow over (r)})where j is a non-negative integer value called the “band index” thatlabels the harmonic modes of the magnetic field {right arrow over(H)}({right arrow over (r)}) in order of increasing angular frequency.

The translational symmetry of the photonic crystal can be used todetermine the functional form of the magnetic fields {right arrow over(H)}_(j) ({right arrow over (r)}). For example, the functional form ofthe magnetic fields {right arrow over (H)}_(j) ({right arrow over (r)})propagating in the photonic crystal 200 are given by the following:{right arrow over (H)} _(j,k) _(∥) _(,k) _(z) ({right arrow over(r)})=exp(i{right arrow over (k)} _(∥)·{right arrow over(ρ)})exp(i{right arrow over (k)} _(z) z){right arrow over (u)} _(j,k)_(∥) _(,k) _(z) (z)  Equation 6:where {right arrow over (p)} is an xy-plane vector, {right arrow over(k)}_(∥) is an xy-plane wave vector, k_(z) is a z-direction wave vectorcomponent, and {right arrow over (u)}_(n,k) _(∥) _(, k) _(z) (Z) is aperiodic function in the z-direction. The exponential term exp(i{rightarrow over (k)}_(∥)·{right arrow over (ρ)}) in Equation 6 arises fromthe continuous translational symmetry of ER propagating through thedielectric layers in the xy-plane. However, the term exp(ik_(z)z){rightarrow over (u)}_(j),k _(∥) _(,k) _(z) (z) in Equation 6 arises fromBloch's theorem and the discrete translational symmetry imposed in thez-direction by the periodicity of the dielectric constant of thephotonic crystal 200, given by:∈({right arrow over (r)})=({right arrow over (r)}+{right arrow over(R)})where {right arrow over (R)}=alź, a is a lattice constant determined bythe regular pattern of the dielectric layers, and/is an integer.

The magnetic fields {right arrow over (H)}_(j,k) _(∥) _(,k) _(z) ({rightarrow over (r)}) are periodic for integral multiples of 2π/α. As aresult, the associated angular frequencies are also periodic:$\begin{matrix}{{{Equation}\quad 7\text{:}}{{\omega_{j}\left( k_{z} \right)} = {\omega_{j}\left( {k_{z} + \frac{m\quad 2\quad\pi}{a}} \right)}}} & \quad\end{matrix}$

Differences in the dielectric pattern of a photonic crystal creates oneor more range of frequencies ω_(j), referred to as “photonic bandgaps,”for which ER is prevented from propagating in the photonic crystal. Thephotonic bandgap also corresponds to an electromagnetic energy range anda range of wavelengths, denoted by λ_(j), for which the differencesbetween the dielectrics prevents ER absorption and ER propagation. Thewavelength λ_(j) of ER transmitted through a photonic crystal is relatedto the angular frequency ω_(j):$\lambda_{j} = \frac{2\quad\pi\quad v}{\omega_{j}}$

where v is the velocity of ER in the photonic crystal. Certain ERfrequency ranges are not transmitted through a photonic crystal becausehigh-frequency harmonic modes tend to concentrate electromagnetic energyin dielectric regions with a low dielectric constant, whilelow-frequency harmonic modes tend to concentrate electromagnetic energyin dielectric regions with a high dielectric constant. Theelectromagnetic energy, W, can be determined from the variationalprinciple as follows:${W\left( \overset{\_}{H} \right)} = {\frac{1}{2\quad\left( {\overset{\_}{H},\overset{\_}{H}} \right)}{\int{{\mathbb{d}\overset{\_}{r}}\quad\frac{1}{ɛ\left( \overset{\_}{r} \right)}{{\bigtriangledown \times {\overset{\_}{H}\left( \overset{\_}{r} \right)}}}^{2}}}}$

where ({right arrow over (H)},{right arrow over (H)})=∫d{right arrowover (r)}{right arrow over (H)}({right arrow over (r)})*{right arrowover (H)}({right arrow over (r)}), and “*” represents the complexconjugate.

The electromagnetic energy is lower for harmonic modes propagating inregions with a high dielectric constant than for modes propagating inregions of a photonic crystal with a low dielectric constant.

The size of and range of frequencies within a photonic bandgap of aone-dimensional photonic crystal depends on the relative differencebetween the dielectric constants of the dielectrics comprising aphotonic crystal. One-dimensional photonic crystals with large relativedifferences between the dielectric constants of the materials comprisingthe photonic crystal have larger photonic bandgaps at higher frequencyranges than photonic crystals with smaller relative differences betweenthe dielectric constants.

FIGS. 4A-4B are hypothetical plots of frequency (ωα/2πc) versus wavevector z-component, k_(z), for a first one-dimensional photonic crystaland a second one-dimensional photonic crystal, respectively. In FIGS.4A-4B, horizontal axes, such as horizontal axis 401, correspond to wavevector z-component k_(z), and vertical axes, such as vertical axis 402,correspond to the frequency. Because the frequencies ω_(j) are periodic,as described above with reference to Equation 7, frequencies (ωjα/2πc)are plotted with respect to wave vector z-component range −π/α and π/αfor angular frequency bands j equal to 1, 2, and 3. The photonicbandgaps are identified by shaded regions 403 and 404. Lines 405, 406,and 407 correspond to the first, second, and third angular frequencybands (j=1, 2, and 3). The width 410 of the photonic bandgap 403, inFIG. 4A, is smaller than the width 412 of the photonic bandgap 404, inFIG. 4B, because the relative difference between the dielectricconstants of the materials comprising the first photonic crystal issmaller than the relative difference between the dielectric constants ofmaterials comprising the second photonic crystal. Also, the photonicbandgap 403 covers a lower range of frequencies than the range offrequencies covered by photonic bandgap 404.

Two-dimensional photonic crystals can be composed of a regular latticeof cylindrical columns fabricated in a dielectric slab. The cylindricalcolumns can be air holes or holes filled with a dielectric materialdifferent from the dielectric material of the photonic slab. FIG. 5illustrates a perspective view of a two-dimensional photonic crystal. InFIG. 5, a photonic crystal 500 is composed of a dielectric slab 501 witha regular lattice of embedded cylindrical columns, such as column 502.The cylindrical columns extend from the top surface to the bottomsurface of the slab 501, as indicated by a cylindrical column 503, andcan be holes filled with air or any other material having a dielectricconstant different from the dielectric constant of the slab 501.Two-dimensional photonic crystals can also be composed of a regularlattice arrangement of cylindrical columns surrounded by a gas or aliquid. FIG. 6 illustrates a two-dimensional photonic crystal 600 havinga regular square lattice of solid cylindrical columns, such as acylindrical column 601, surrounded by fluid, such as gas or liquid, witha dielectric constant different from the cylindrical columns.

Two-dimensional photonic crystals polarize ER propagating in theperiodic plane of the photonic crystal, and the electric and magneticfields can be classified into two distinct polarizations: (1) thetransverse electric-field (“TE”) modes; and (2) the transversemagnetic-field (“TM”) modes. The TE have {right arrow over (H)}({rightarrow over (ρ)}) directed normal to the periodic plane of the photoniccrystal and {right arrow over (E)}({right arrow over (ρ)}) directed inthe periodic plane of the photonic crystal, while the TM have {rightarrow over (E)}({right arrow over (ρ)}) directed normal to the periodicplane of the photonic crystal and {right arrow over (H)}({right arrowover (ρ)}) directed in the periodic plane of the photonic crystal. FIGS.7A-7B illustrate propagation of TE and TM modes in the two-dimensionalphotonic crystal shown in FIG. 5. The periodic plane of the photoniccrystal 500 lies in the xy-plane, the cylindrical columns are parallelto the z-direction, and ER propagates through the photonic crystal 500in the y-direction. In FIG. 7A, an oscillating curve 701 represents the{right arrow over (H)}({right arrow over (ρ)}) mode directed normal tothe xy-plane, and an oscillating curve 702 represents the orthogonal{right arrow over (E)}({right arrow over (ρ)}) mode directed in thexy-plane. In FIG. 7B, an oscillating curve 703 represents the {rightarrow over (E)}({right arrow over (ρ)}) mode directed normal to thexy-plane, and an oscillating curve 704 represents the orthogonal {rightarrow over (H)}({right arrow over (ρ)}) mode directed in the xy-plane.

FIG. 8 illustrates a photonic band structure of TM and TE modes of an ERpropagating in the photonic crystal shown in FIG. 5. In FIG. 8, avertical axis 801 represents the angular frequency of ER propagating inthe photonic crystal 500, and a horizontal axis 802 represents the ERpropagation paths between lattice points labeled Γ, M, and K in aphotonic crystal segment 803 of the photonic crystal 500, shown in FIG.5. Solid lines, such as solid line 804, represent TM modes, and dashedlines, such as dashed line 805, represent the TE modes. A shaded region806 identifies a photonic bandgap in which neither the TE nor TM modesare permitted to propagate in the photonic crystal 500.

The width and the frequency range covered by photonic bandgaps intwo-dimensional photonic crystal slabs, such as the photonic bandgap806, depends on the periodic spacing of the cylindrical columns,represented by lattice constant a, and the relative difference betweenthe dielectric constant of the slab and the dielectric constant of thecylindrical columns. Also, the frequency range covered by photonicbandgap 806 can be shifted to a higher frequency range for largerrelative differences between the dielectric constant of the slab and thedielectric constant of the cylindrical columns, while the photonicbandgap 806 can be shifted to a lower frequency range for smallerrelative differences between the dielectric constant of the slab and thedielectric constant of the cylindrical columns.

Electron beam or nanoimprint lithography followed by chemical etching,or other processing methods, can be used to fabricate cylindricalcolumns in a suitable two-dimensional dielectric slab. In addition,two-dimensional photonic crystals can be designed to reflect ER within aspecified frequency band. As a result, a two-dimensional photoniccrystal can be designed and fabricated as a frequency-band stop filterto prevent the propagation of ER having frequencies within the photonicbandgap of the photonic crystal. Generally, the size and relativespacing of cylindrical columns control which wavelengths of ER areprohibited from propagating in the two-dimensional photonic crystal.However, defects can be introduced into the lattice of cylindricalcolumns to produce particular localized optical components. Inparticular, a point defect, referred to as a “resonant cavity,” can befabricated to provide a resonator that temporarily traps a narrow rangeof frequencies or wavelengths of ER. A line defect, referred to as a“waveguide,” can be fabricated to transmit ER with frequency ranges orwavelengths that lie within a frequency range of a photonic bandgap. Asa result, a three-dimensional photonic crystal slab can be thought of astwo-dimensional crystal having a refractive index n that depends on thethickness of the slab.

FIG. 9 illustrates an example of a photonic crystal with two resonantcavities and a waveguide. A resonant cavity can be created in atwo-dimensional photonic crystal slab by omitting, increasing, ordecreasing the size of a select cylindrical column. For example, aresonant cavity 901 is created in a photonic crystal 900 by omitting acylindrical column, as indicated by the empty region surrounded by adashed-line circle. Resonant cavities 901 and 905 are surrounded byeffectively reflecting walls that temporarily trap ER in the frequencyrange of the photonic bandgap. Resonant cavities can channel ER within anarrow frequency band in a direction perpendicular to the plane of thephotonic crystal. For example, the resonant cavity 901 can traplocalized TM modes and TE modes within a narrow frequency band of thephotonic bandgap. Unless the photonic crystal 900 is sandwiched betweentwo reflective plates or dielectrics that create total internalreflection, the ER resonating in the resonant cavity 901 can escape inthe direction perpendicular to the periodic plane of the photoniccrystal 900. Each resonant cavity has an associated quality (“Q”) factorthat provides a measure of how many oscillations take place in a cavitybefore the ER resonating in the resonant cavity diffuse into the regionsurrounding the resonant cavity.

Waveguides are optical transmission paths that can be used to direct ERwithin a particular frequency range of the photonic bandgap from a firstlocation in a photonic crystal to a second location in the photoniccrystal. Waveguides can be fabricated by changing the diameter ofcertain cylindrical columns within a column or row of cylindricalcolumns, or by omitting rows of cylindrical columns. For example, in thephotonic crystal 900, a dielectric waveguide 902 is created by omittingan entire row of cylindrical columns during fabrication of the photoniccrystal 900, as indicated by the empty region between dashed lines 903and 904. The dielectric waveguide 902 transmits ER with wavelengths λ₀and λ₁ along a single path. Networks of branching waveguides can be usedto direct ER in numerous different pathways through the photoniccrystal. The diameter of an optical signal propagating along a waveguidecan be as small as □/3n, where n is the refractive index of thewaveguide, while a harmonic mode volume of a resonant cavity can be assmall as 2 (□/3n)³.

Waveguides and resonant cavities may be less than 100% effective inpreventing ER from escaping into the area immediately surrounding thewaveguides and resonant cavities. For example, ER within a frequencyrange in the photonic bandgap propagating along a waveguide also tendsto diffuse into the region surrounding the waveguide. ER entering thearea surrounding a waveguide or a resonant cavity experiences anexponential decay in amplitude, a process referred to as “evanescence.”As a result, a resonant cavity can be located within a short distance ofa waveguide to allow certain wavelengths of ER carried by the waveguideto be extracted by the resonant cavity. In effect, resonant cavities arefilters that can be used to extract a fraction of a certain wavelengthof ER propagating in the waveguide. Depending on a resonant cavity Qfactor, an extracted ER can remain trapped in a resonant cavity andresonate for a time before evanescing into the surroundings orbackscattering into the waveguide. For example, in FIG. 9, the resonantcavity 901 is located too far from the waveguide 902 to extract a modewith particular wavelength of ER. However, the resonant cavity 905 isable to extract a fraction of ER with wavelength λ₃ propagating alongthe waveguide 902. Thus, a smaller fraction of ER with wavelength λ₃ maybe left to propagate in the waveguide 902 along with ER of wavelengthsλ₁ and λ₂.

FIG. 10 is a hypothetical plot of frequency versus the magnitude of wavevector {right arrow over (k)}_(∥) for the waveguide of the photoniccrystal shown in FIG. 9. In FIG. 10, shaded regions 1001 and 1002represent projected first and second band structures of the photoniccrystal 900 in the absence of the waveguide 902, shown in FIG. 9. Aregion 1003 identifies the photonic bandgap created by the photoniccrystal 900. Line 1004 identifies a band of frequencies permitted topropagate in the waveguide 902. The number of frequency bands permittedto propagate in waveguide 902 can be increased by increasing the size ofthe waveguide 902.

For three-dimensional photonic crystals, the three-dimensional latticeparameters, the difference between dielectric constants, and thedimensions of the inclusions determine the frequency range of photonicbandgaps. Waveguides and resonant cavities can also be fabricated inthree- dimensional photonic crystals by selectively removing or changingthe dimensions of certain inclusions.

An Overview of Encoding Data in Electromagnetic Radiation

A bit is a basic unit of information in computational systems and isequivalent to a choice between two alternatives, such as “yes” and “no,”or “on” and “off.” The two states for a bit are typically represented bythe numbers 1 or 0. Information can be encoded in an electromagneticwave by modulating the electromagnetic wave amplitude frequency, orphase. The modulated electromagnetic waves can then be transmitted overlarge distance in optical fibers, waveguides, or through free space, anddecoded by a demodulator. However, most electromagnetic waveinteractions with matter result from the electric field component ratherthan the magnetic field component, because the amplitude of the magneticfield is smaller than the amplitude of the electric field by the factor1/c, where c represents the speed of light. As a result, and for thesake of simplicity, an electromagnetic wave can be represented by theelectric field component:E(z,t)=E ₀ cos(zk−ωt)where the electric field propagates in the z direction, ω is angularfrequency, k is a wavevector ω/c, t is time, and E₀ is the amplitude.FIG. 11A is a plot of an electromagnetic wave as a function of time anda fixed observation point. In FIG. 11A, horizontal line 1102 is a timeaxis, vertical line 1104 is the amplitude E₀, and curve 1106 representsthe electric field E(z,t). The period T is the time it takes for theelectromagnetic signal to complete a cycle. The angular frequency ω is2πυ, where υ is the frequency, or number of times, the electromagneticfield completes a cycle per unit of time.

Amplitude modulation is used to encode information by changing thestrength or magnitude of the amplitude of the electromagnetic signal.FIG. 11 B illustrates an example of an amplitude modulatedelectromagnetic signal encoding of the bits “0” and “1.” In FIG. 11B, abit corresponds to four consecutive cycles of the signal, where thecycles 1108 with a small amplitude 1110 corresponds to the bit “0,” andthe cycles 1112 with a relatively large amplitude 1114 corresponds tothe bit “1.” Frequency modulation is used to encode information byvarying the frequency of the electromagnetic signal. FIG. 11Cillustrates an example of a frequency modulated electromagnetic signalencoding of the bits “0” and “1.” In FIG. 11C, the four consecutivecycles 1116 correspond to the bit “1,” and the two consecutive cycles1118 corresponds to the bit “0.” Phase modulation is used to encodeinformation by shifting the phase of the electromagnetic signal asfollows:E(z,t)=E ₀ cos(zk−ωt+φ)where φ represents a phase shift. A phase shift corresponds to a shiftin the waveform of the electromagnetic signal. For example, FIG. 11Dillustrates a curve 1120 that includes a ¼ cycle phase shift of a curve1122. FIG. 11E illustrates an example of a phase modulatedelectromagnetic signal encoding of the bits “0” and “1.” In FIG. 11E,the cycles 1124 corresponds to a bit “1,” and the cycles 1126 includes a½ cycle phase shift that corresponds to the bit “0.”

EMBODIMENTS OF THE PRESENT INVENTION

FIG. 12 illustrates a photonic-interconnection-based compute clusterthat represents one of many embodiments of the present invention.Compute cluster 1200 is composed of groups 1201-1204, a switch fabric1206, a clock frame 1208, and a tree of branching optical transmissionpaths represented by branching lines, such as line 1210. The groups1201-1204 are each composed of one or more nodes described below withreference to FIG. 14. In FIG. 12, an optical signal source (not shown)generates an optical signal comprising eight independent electromagneticwaves called “frequency channels,” each frequency channel having adifferent wavelength that are represented by λ₁, λ₂, λ₃, λ₄, λ₅, λ₆, λ₇,and λ₈. The optical signal enters the compute cluster 1200 in an opticaltransmission path 1212 in the direction identified by the directionarrow 1214. The optical transmission path 1212 leads to other branchingoptical transmission paths that transmit all eight of the frequencychannels to the groups 1201-1204. A number of the frequency channels canthen be encoded with data by the nodes of each group and redistributedto different nodes by the switch fabric 1206. The clock frame 1208provides a clock signal that can be used to synchronize operation of thecomponents comprising the compute cluster 1200.

The optical transmission paths can be optical fibers, coaxial cables,waveguides in a photonic crystal, or any combination of optical fibers,coaxial cables, and waveguides. A single optical transmission path cantransmit numerous independent frequency channels, each frequency channelrepresenting an independent bus. FIGS. 13A-13C illustrate examples ofwaveguides in a two-dimensional, photonic-crystal-based interconnectionthat can be used to transmit and distribute an optical signal to eachgroup of a compute cluster that represents one of many embodiments ofthe present invention. In FIGS. 13A-13C, waveguides are located within alattice of cylindrical columns, such as cylindrical column 1301 shown inFIG. 13A. The cylindrical columns can be air holes or holes filled witha dielectric material different from the dielectric material of thephotonic crystal slab. Two-dimensional photonic interconnections mayhave cylindrical column diameters and lattice constants on the order ofa few hundred nanometers or less. The diameter and pattern ofcylindrical columns, and the dielectric material in or surroundingcylindrical columns, can be selected to create photonic bandgaps thateffectively confine the optical signal to the waveguides. FIG. 13Aillustrates a straight-line waveguide that can be used to transmit theoptical signal comprising frequency channels λ₁, λ₂, λ₃, λ₄, λ₅, λ₆, λ₇,and λ₈ along a straight path, such as the optical transmission path 1212in FIG. 12. FIG. 13B illustrates a bent waveguide that can be used toconfine and direct the path of the optical signal, such as bent opticaltransmission path 1220 in FIG. 12. FIG. 13C illustrates a Y-shapedwaveguide that can be used to transmit the same optical signal into twodifferent waveguides, such as the Y-shaped optical transmission path1222 in FIG. 12 that transmits the same optical signal to groups 1201and 1202.

The frequency channels transmitted by the optical transmission paths inFIG. 12 are divided into a first set of frequency channels and a secondset of frequency channels. The first set of frequency channels λ₁, λ₂,λ₃, and λ₄ are modulated by the nodes in the groups 1201-1204 to encodedata and are transmitted along with the second set of frequencychannels, λ₅, λ₆, λ₇, and λ₈ to the switch fabric 1206 in opticaltransmission paths 1216-1219. FIG. 14 illustrates an example of a groupthat represents one of many embodiments of the present invention. InFIG. 14, group 1400 is composed of four nodes 1401-1404, and an opticaltransmission path 1406. Each node can be a processor, memory, computerserver, storage server, an external network connection, a datatransmitting device or any electrical circuit or mosaic of electricalcircuits having either microscale or nanoscale dimensions. Opticaltransmission path 1406 can be a waveguide in a photonic crystal thattransmits the optical signal comprising the frequency channels λ₁, λ₂,λ₃, λ₄, λ₅, λ₆, λ₇, and λ₈ to the nodes 1401-1404.

Each node in the group 1400 is located near a photonic-crystal-basedwriter, represented by downward directional arrows 1408-1411, thatextracts and encodes data in a specific frequency channel of the firstset of frequency channels. For example, photonic crystal-base writer1408 extracts frequency channel λ₁, encodes data generated by the node1401 in frequency channel λ₁ to obtain modulated frequency channel λ₁^(g), and inserts the modulated frequency channel λ₁ ^(g) into thewaveguide 1406, where the superscript, g, identifies the group. Forexample, the superscript g can be “0,”“1,”“2,” or “3,” which are used toidentify the groups shown in FIG. 12. The modulated frequency channelsλ₁ ^(g), λ₂ ^(g), λ₃ ^(g), and λ₄ ^(g) and the second set of unmodulatedfrequency channels λ₅, λ₆, λ₇, and λ₈ are transmitted by the waveguide1406 to the switch fabric 1206 in FIG. 12.

Each node is also located near a photonic-crystal-based reader,identified by upward directional arrows 1412-1415, that extracts thedata encoded in the frequency channels λ ₅, λ ₆, λ ₇, and λ ₈, where thebar represents frequency channels transmitted to the group 1400 by theswitch fabric 1206, that have each been encoded with data by the nodesof different groups in the compute cluster 1200. In order for the nodesto communicate with the photonic-crystal-based writers and readers, eachnode includes an interface, such as interface 1418, comprising amultiplexer/demultiplexer that transmits electrical signals between theinternal components of the node and the corresponding attachedphotonic-crystal-based writer and reader described below.

Photonic-crystal-based writers encode data in a specific frequencychannel by extracting the specific frequency channel transmitted by awaveguide, modulating the extracted frequency channel as directed by anode, and inserting the modulated frequency channel into the waveguideto be read by a different node elsewhere in the compute cluster. FIG.15A illustrates a photonic-crystal-based writer that encodes data in aspecific frequency channel of an optical signal that represents one ofmany embodiments of the present invention. Photonic-crystal-based writer1500 encodes data in frequency channel λ₁. The writer 1500 includes adrop filter 1502, a local waveguide 1504, a modulator 1506, an addfilter 1508, and a waveguide 1406. The drop filter 1502 is a resonantcavity that extracts and confines the frequency channel λ₁ viaevanescent coupling from the waveguide 1406. The frequency channel λ₁evanesces from the drop filter 1502 into the local waveguide 1504 thenevanesces from the local waveguide 1504 into the modulator 1506. Themodulator 1506 is a resonant cavity, described below with reference toFIGS. 16A-16C, that modulates the frequency channel λ₁ in accordancewith encoding instructions received by the node 1401 to generate themodulated frequency channel λ₁ ^(g). The add filter 1508 is a resonantcavity that receives the modulated frequency channel λ₁ ^(g) viaevanescent coupling from the modulator 1506 and inserts the modulatedfrequency channel λ₁ ^(g) via evanescent coupling into the waveguide1506.

Photonic-crystal-based readers extract a specific modulated frequencychannel written and sent by a different node in the compute cluster.FIG. 15B illustrates a photonic-crystal-based reader that extracts dataencoded in a specific frequency channel of an optical signal thatrepresents one of many embodiments of the present invention.Photonic-crystal-based reader 1550 extracts encoded frequency channel λ₅. The reader 1550 includes a drop filter 1552 and a detector 1554. Thedrop filter 1552 extracts and confines the frequency channel λ ₅ viaevanescent coupling from the waveguide 1406. The frequency channel λ ₅evanesces from the drop filter 1552 into the demodulator 1554. Thedemodulator 1554 is a resonant cavity, described below with reference toFIGS. 16A-16C, that includes photodetectors for extracting the digitalinformation carried by the modulated frequency channel λ ₅.

In general, the drop filters and the add filters ofphotonic-crystal-based writers and photonic-crystal-based readers arepositioned within a range of evanescent fields emanating from awaveguide. Both drop and add filter diameters and distances to thewaveguide can be selected so that the associated resonant cavities areresonators for specific wavelengths carried by the waveguide. Forexample, the resonant cavities associated with the drop filters 1502 and1552, shown in FIGS. 15A-15B, are dimensioned and positioned near thewaveguide 1406 to extract and confine the frequency channels λ ₁ and λ₅, respectively. The add filter 1508, in FIG. 15A, is dimensioned andlocated near the waveguide 1406 to insert the modulated frequencychannel λ₁ ^(g) into the waveguide 1406. The local waveguide 1504, shownin FIG. 15A, is located near the modulator 1506 so that a large fractionof the frequency channel transmitted by the local waveguide 1504 can becoupled into the modulator 1506. The modulator 1506 is also dimensionedand positioned to create a strong resonant coupling with the add filter1508, so that the add filter 1508 can insert the modulated frequencychannel λ₁ ^(g) into the waveguide 1406. The dielectric constant of thephotonic crystal slab, and the spacing and/or size of the lattice ofcylindrical columns surrounding each resonator cavity can be selected sothat the drop filters can only extract certain frequency channels. Inorder to provide strong couplings between a waveguide and drop and addfilters, the resonant cavities can be fabricated with high Q factors,such as a Q factor of about 1,000 or larger.

Drop filters and add filters can be fabricated using a variety ofdifferent defects in a photonic crystal. FIG. 16A illustrates a resonantcavity that can be used as either a drop filter or an add filter thatrepresents one of many embodiments of the present invention. In FIG.16A, a resonant cavity 1602 can be created by omitting a cylindricalcolumn within a regular triangular pattern of cylindrical columns in aphotonic crystal slab 1604. The diameter of the resonant cavity 1602 andthe pattern and diameter of cylindrical columns surrounding resonantcavity 1602, such as cylindrical column 1606, can be selected toeffectively prevent a specific frequency channel from evanescing intothe surrounding photonic crystal slab 1604. A resonant cavity may alsobe fabricated using a cylindrical column having a diameter differentfrom the diameter of surrounding cylindrical columns, and/or filling acylindrical column with a dielectric material different from thedielectric material of the surrounding cylindrical columns. The photoniccrystal slab 1604 is located on top of a glass substrate 1608 and iscomposed of a positively doped semiconductor layer 1610, an insulatinglayer 1612 located on top of the semiconductor layer 1610, and anegatively doped semiconductor layer 1614 located on top of theinsulating layer 1612. The layers 1614, 1612, and 1610 compose a singlelayer referred to as a “p-i-n” layer. The dopant concentrations of thep-i-n layers can be any combination of Si, SiO, SiO₂, InGaAs, or anyother suitable dopant.

Demodulators and modulators can be fabricated at resonant cavities froma variety of different materials. FIG. 16B illustrates a firstconfiguration of a demodulator/modulator that represents one of manyembodiments of the present invention. A demodulator/modulator 1616 canbe fabricated using a resonant cavity, such as the resonant cavity 1602,sandwiched between two electrodes 1620 and 1622. The electrode 1620 isin contact with the semiconductor layer 1610, and the electrode 1622 isin contact with the semiconductor layer 1614. In order for thedemodulator/modulator 1616 to operate as a demodulator, the electrodes1620 and 1622 collect a varying electrical current generated byvariations in the intensity, phase, and/or amplitude of a frequencychannel resonating in the resonant cavity 1602. The varying electricalcurrent represents a data stream that can be transmitted from theelectrodes 1620 and 1622 to a node interface, such as the interface 1418in FIG. 14, via signal lines (not shown). The semiconductor layers 1610and 1614 may have different dopant concentrations or dopant types sothat the demodulator/modulator 1616 can operate as a modulator byvarying a voltage applied to the electrodes 1620 and 1622. The appliedvoltage is provided by a node interface to modulate a frequency channelresonating in the resonant cavity 1602 by changing the phase and/oramplitude of the frequency channel.

FIG. 16C illustrates a second configuration of a demodulator/modulatorthat represents one of many embodiments of the present invention.Demodulator/modulator 1626 includes the resonant cavity 1602, and twoelectrodes 1628 and 1630 that are both located under the resonant cavity1602. The layer 1604 can be composed of the p-i-n layers, describedabove with reference to FIG. 16A, or a single layer, such as a singlelayer of lithium niobate, LiNbO₃. A demodulator/modulator 1626 operatesas a demodulator by collecting a varying electrical current inelectrodes 1628 and 1630 that is generated by variations in theintensity, phase, and/or amplitude of a frequency channel resonating inthe resonant cavity 1602. The demodulator/modulator 1626 operates as amodulator by varying a voltage applied to the electrodes 1628 and 1630that, in turn, changes the dielectric constant of the dielectricmaterials in the resonant cavity 1602 causing a phase and/or amplitudechange in a frequency channel resonating in the resonant cavity 1602.

The intrinsic capacitance in demodulator electrode detectors is oftenlow enough that fluctuations in current due to noise generated bythermal agitation of electrons in a conductor, called “Johnson noise,”may be insignificant. As a result, statistics associated with an opticalsignal source dominate the bit error rate (“BER”) arising in the serialdigital signal corresponding to the output from the detector. Forexample, a Poisson distribution of an optical signal having 30 photonsper bit is sufficient to achieve a BER of less than 10⁻¹³. Incorporatinga doped region into a resonant cavity with a Q factor of 10 to 100 maycompensate for the reduced absorption. With an appropriate choice of Qfactor to impedance-match, the optical input losses of the cavity to theinternal absorption loss of the detector may increase detectionefficiency. For example, an increase in the detection efficiency ofabout 50% may be achieved.

Similar considerations can be applied to the design of a resonant cavityenhanced (“RCE”) modulator using electro-optic techniques. Modulationdepths as high as 50% may be achieved for a resonant cavity with a Qfactor greater than about 1,000. Although other physical effects can beemployed, such as variations in the free carrier plasma index,electro-optic modulation can be used with a potential difference ofabout 30 mV applied across a gap of about 300 nm to produce an electricfield of 1 kV/cm, which is sufficient to generate a refractive indexchange as large as 0.001 in a wide variety of linear dielectricmaterials.

After the groups have encoded data in half of the frequency channelstransmitted by a waveguide, the switch fabric 1206, in FIG. 12, employsthe unencoded frequency channels to encode data directed to other nodeswithin the compute cluster. FIG. 17 illustrates a schematicrepresentation of the switch fabric 1206, shown in FIG. 12, thatrepresents one of many embodiments of the present invention. The switchfabric 1206 receives modulated and unmodulated frequency channelstransmitted from the groups 1201-1204 in the waveguides 1216-1219,respectively. Photonic-crystal-based readers, represented by boxes withupward directed arrows, extract the modulated frequency channelstransmitted in waveguides 1216-1219, as described above with referenceto FIG. 15B. The photonic-crystal-based readers convert the extractedfrequency channels into continuous electrical bit streams. Anelectronic-based switch fabric 1706 directs the bit streams to nodes inthe compute cluster 1200, in FIG. 12. For example,photonic-crystal-based readers 1701-1704 extract the modulated frequencychannels λ₁ ⁰, λ₂ ⁰, λ₃ ⁰, and λ₄ ⁰ from waveguide 1216 and convert themodulated frequency channels into electronic input bit streams e₁ ⁰, e₂⁰, e₃ ⁰, and e₄ ⁰, respectively. The input bits streams e₁ ⁰, e₂ ⁰, e₃⁰, and e₄ ⁰ are transmitted in signal lines, such as signal line 1705,to the electronic-based switch fabric 1706, where the bit streams arepartitioned, assembled, and directed to particular nodes of the computecluster, as described below with reference to FIGS. 17-18. The outputbit streams e₅ ⁰, e₆ ⁰, e₇ ⁰, and e₈ ⁰ represent assembled bit streamsoutput by the electronic-based switch fabric 1706.Photonic-crystal-based writers, represented by boxes with downwarddirected arrows, encode the information in the output bit streams bymodulating the unmodulated frequency channels λ₅, λ₆, λ₇, and λ₈transmitted in each waveguide. For example, photonic-crystal-basedwriters 1707-1710 encode the data contained in the output bit steams e₅⁰, e₆ ⁰, e₇ ⁰, and e₈ ⁰ by modulating the unmodulated frequency channelsλ₅, λ₆, λ₇, and λ₈ to obtain modulated frequency channels λ₅ ⁰, λ₆ ⁰, λ₇⁰, and λ₈ ⁰, respectively. The modulated frequency channels λ₁ ⁰, λ₂ ⁰,λ₃ ⁰, and λ₄ ⁰ are transmitted in waveguide 1216 to the nodes of group1201, in FIG. 12.

FIG. 18 illustrates an implementation of the electronic-based switchfabric 1706 shown in FIG. 17 that represents one of many embodiments ofthe present invention. In FIG. 18, the bit streams are passed to acyclic permutation network 1802. The cyclic permutation network 1802 iscomposed of a network of signal lines that are used to distribute thebits streams to virtual output queues, as described below with referenceto FIGS. 19A-19B. The virtual output queues, such as virtual outputqueue 1804, receive a continuous bit stream from the cyclic permutationnetwork 1802 and convert the continuous bit stream into parallel wordsstreams that are stored as packets. Each packet includes a group andnode address that enables the packet to be directed to a node. Certainpackets in each virtual queue are multiplexed in order to assemblecontinuous bit streams that are transmitted to a cyclic permutationnetwork 1806. The buffering and multiplexing performed by each virtualoutput queue is described below with reference to FIG. 19. As packetsare output from a virtual output queue, the cyclic permutation network1806 directs the assembled bit streams to appropriate nodes identifiedby the group and node addresses. The operation of the cyclic permutationnetworks 1802 and 1806, and the virtual output queues, are synchronizedin accordance with the clock signal provided by clock frame 1208,described above with reference to FIG. 12.

FIG. 19A illustrates an example of a cyclic permutation network thatrepresents one of many embodiments of the present invention. Cyclicpermutation network 1900 includes inputs 1901-1904 that receive bitstreams, e₀, e₁, e₂, and e₃. Each input is connected to a set of fourdifferent signal lines, such as signal line 1905, that transmit the samebit stream to all four identically configured multiplexers 1906-1909.Each signal line in a set of four different signal lines is connected toa different input address of the multiplexers 1906-1909 by a cyclicpermutation shift in the input address order. For example, bit stream e₀is transmitted to the first input address of the multiplexer 1906, thesecond input address of the multiplexer 1907, the third input address ofmultiplexer 1908, and the fourth input address of the multiplexer 1909.At each timeslot of the clock signal, a specific input address is sentto the multiplexers 1906-1909, causing the bit stream input at the sameinput address to be output. For example, the address for the first inputaddress of the multiplexers 1906-1909 is sent to each multiplexer at thesame time causing the bit streams e₀, e₃, e₂, and e₁, to be outputsimultaneously from the multiplexers 1906-1909, respectively. FIG. 19Billustrates four possible cyclic permutation outputs that can begenerated by the cyclic permutation network 1900 shown in FIG. 19A. Thecyclic permutation network is not limited to four inputs. In alternateembodiments, cyclic permutation networks can be fabricated to permute Mdifferent bit streams to M identically configured multiplexers, eachmultiplexer having M different input addresses, where M is any positiveinteger.

FIG. 20 illustrates an implementation of an exemplary virtual outputqueue that represents one of many embodiments of the present invention.In FIG. 20, virtual output queue 2000 receives a continuous bit streamthat is represented by directional arrow 2002. A converter 2004partitions the bit stream 2002 into packets that are transmitted to ademultiplexer 2006. The converter 2004 also assigns an address in theform a additional bit stream to each packet that identifies a node andgroup for which each packet is to be sent for processing. Thedemultiplexer 2006 receives the packets and buffers each packet into avirtual output queue according to the address assigned to each packet.For example, the packets, such as packet 2010, are buffered in a buffer2012, and each packet has the same node and group address. A multiplexer2014 cycles through each of the virtual queues, one queue per timeslotof the clock signal, dequeues packets with the same node and groupaddress, then transmits the dequeued packets to a converter 2016. Theconverter 2016 assembles the dequeued packets into a continuous bitstream, represented by directional arrow 2018, and transmits thecontinuous bit stream to the appropriate node and group identified bythe address for processing.

Although the present invention has been described in terms of particularembodiments, it is not intended that the invention be limited to theseembodiments. Modifications within the spirit of the invention areapparent to those skilled in the art. For example, in an alternateembodiment of the present invention, the photonic interconnectionsignaling systems can instead be employed to implement quantum systemsthat manipulate quantum states, such as qubits, qudits, or qunits. In analternate embodiment of the present invention, the optical signalstransmitted by waveguides can represent quantum information, and nodeinterfaces can route selected optical signals to nanoscale electroniccircuits, or convert the optical signals into a form suitable for thenanoscale electronic circuits. In an alternate embodiment of the presentinvention, photonic-interconnection-based compute cluster can be used inquantum information processing using optical pulse control ofelectron-spin-based semiconductor quantum computers. In semiconductorquantum computers, each qubit can be represented by a spin state of asingle electron or a quantum dot. A quantum dot represents the presenceor absence of a single electron. A quantum dot can be created using anysubstance that allows for detection of a single electron, such as asemiconductor, a metal, an atom, or a molecule. Single-qubit andtwo-qubit logical operations are implemented by applying optical controlpulses to particular quantum dots. Semiconductor quantum computerscombine quantum optics and spintronics, which includes precise controlprovided by lasers, the availability of resonance-fluorescencemeasurements, and the long spin coherence times of electrons insemiconductors. An application of the architecture shown in FIG. 12 canbe applied to an electron-spin-based semiconductor quantum computer tosend a laser control pulse that a drop-filter extracts for applicationto a target quantum dot, represented by a node in FIG. 14. As a result,the target quantum dot can perform a logic operation on the qubit, orbetween the qubit and a qubit in a neighboring quantum dot.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that the specificdetails are not required in order to practice the invention. Theforegoing descriptions of specific embodiments of the present inventionare presented for purposes of illustration and description. They are notintended to be exhaustive of or to limit the invention to the preciseforms disclosed. Obviously, many modifications and variations arepossible in view of the above teachings. The embodiments are shown anddescribed in order to best explain the principles of the invention andits practical applications, to thereby enable others skilled in the artto best utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the followingclaims and their equivalents:

1. A compute cluster comprising: a photonic interconnection having oneor more optical transmission paths for transmitting independentfrequency channels within an optical signal to each node in a set ofnodes; one or more photonic-interconnection-based writers, each writerassociated with a particular node, and each writer encoding informationgenerated by the node into one of the independent frequency channels; aswitch fabric that directs the information encoded in the independentfrequency channels to one or more nodes in the compute cluster; and oneor more photonic-interconnection-based readers, each reader associatedwith a particular node, and each reader extracting the informationencoded in the independent frequency channels directed to the node forprocessing.
 2. The compute cluster of claim 1 further comprising a clocksignal source.
 3. The compute cluster of claim 2 wherein the switchfabric further comprises one or more cyclic permutation networks fordistributing continuous bit streams of information and one or morevirtual output queues for queuing packets of information into one ormore buffers.
 4. The compute cluster of claim 1 wherein the nodesfurther comprise any one of a processor, memory, computer server,storage server, an external network connection, a data transmittingdevice or any electrical circuit or mosaic of electrical circuits. 5.The compute cluster of claim 4 where the electrical circuits havemicroscale dimensions or nanoscale dimensions.
 6. The compute cluster ofclaim 1 wherein the photonic interconnection further comprises atwo-dimensional photonic crystal with waveguides, a number ofindependent optical fibers, or free space.
 7. The compute cluster ofclaim 1 wherein the photonic-interconnection-based writer furthercomprises: a drop filter that extracts a frequency channel from theoptical transmission path, a local waveguide that extracts the frequencychannel from the drop filter and transmits the frequency channel, amodulator that extracts the frequency channel from the local waveguideand produces a modulated frequency channel by modulating the frequencychannel, and an add filter that extracts the modulated frequency channeland inserts the modulated frequency channel into the opticaltransmission path.
 8. The compute cluster of claim 1 wherein thephotonic-interconnection-based reader further comprises: a drop filterthat extracts a frequency channel from the optical transmission path,and a detector that extracts the frequency channel from the drop filterand demodulates the information encoded in the frequency channel.
 9. Aswitch fabric comprising: one or more optical transmission paths thateach transmit a first set of independent frequency channels and a secondset of independent frequency channels, the first set of independentfrequency channels encoding information output by nodes in a computecluster; one or more photonic-interconnection-based readers, each readerextracting a frequency channel in the first set of independent frequencychannels and encoding the information into one or more electronic inputbit streams; an electronic-based switch fabric that partitions andassembles the electronic input bit streams into electronic output bitstreams, each electronic output bit stream directed to a particular nodein the compute cluster; and one or more photonic-interconnection-basedwriters, each encoding the information encoded in the electronic outputbit streams into frequency channels in the second set of independentfrequency channels.
 10. The switch fabric of claim 9 further comprisinga clock signal source.
 11. The switch fabric of claim 10 wherein theswitch fabric further comprises one or more cyclic permutation networksfor distributing continuous bit streams of information and one or morevirtual output queues for queuing packets of information into one ormore buffers.
 12. The switch fabric of claim 9 wherein the nodes furthercomprise any one of a processor, memory, computer server, storageserver, an external network connection, a data transmitting device orany electrical circuit or mosaic of electrical circuits.
 13. The switchfabric of claim 12 where the electrical circuits have microscaledimensions or nanoscale dimensions.
 14. The switch fabric of claim 9wherein the photonic interconnection further comprises a two-dimensionalphotonic crystal with waveguides, a number of independent opticalfibers, or free space.
 15. A method for transmitting data between nodesin a compute cluster, the method comprising: providing a photonicinterconnection having one or more optical transmission paths that leadto each node in the computer cluster; providing an optical signalcomposed of a first of independent frequency channels and a second setof independent frequency channels, both sets of independent frequencychannels are transmitted in the optical transmission paths to the nodes;encoding information generated by the nodes into the frequency channelsin the first set of independent frequency channels; transmitting theencoded information to a switch fabric that partitions and assembles theinformation encoded in the first set of frequency channels and encodesthe assembled information into the frequency channels of the second setof independent frequency channels; and directing the second set ofindependent frequency channels to the nodes so that the assembledinformation can be processed.
 16. The method of claim 15 furthercomprising providing a clock signal that synchronizes transmission ofinformation between nodes in the compute cluster.
 17. The method ofclaim 16 wherein providing the switch fabric further comprises providingone or more cyclic permutation networks for distributing continuous bitstreams of data and one or more virtual output queues for queuingpackets of data into one or more buffers.
 18. The method of claim 15wherein the node further comprises any one of a processor, memory,computer server, storage server, an external network connection, a datatransmitting device or any electrical circuit or mosaic of electricalcircuits.
 19. The method of claim 18 where the electrical circuits havemicroscale dimensions or nanoscale dimensions.
 20. The method of claim15 wherein the photonic interconnection further comprises atwo-dimensional photonic crystal with waveguides, a number ofindependent optical fibers, or free space.